For molded articles with a mineral bond, used primarily in construction as lightweight fiber boards, an organic or inorganic fibrous material can be used as a base material. For example, wood fiber, wood chips, sawdust, pulpwood and other lignocelluloses, mineral fibers, such as slag wool, glass fibers, as well as plastic chips and fibers can be used. However, the base material can also consist of lightweight materials of various types, such as, for example, those on the basis of expanded silicates (e.g. perlite) or expanded micas (e.g. vermiculite), expanded clays or expanded glass, or on an organic basis, for which among others granules of foamed plastics (e.g. preformed polystyrene) are suitable as fillers. The lightweight materials can be used for reinforcement purposes, also in combination with the above mentioned fibrous materials.
The mineral binders which are used primarily in the building material industry for the production of molded articles are portland cement and magnesia cement. In the production of boards and molded articles in individual molds it is irrelevant which binder is used.
Quite different, however, are the conditions if the production of the boards is to be effected continuously in a molding duct formed of endless moving belts, particularly steel belts, also called a belt-molding machine. In the belt-molding machine, the provisional setting of the molded material must be effected so fast that the endless rope emerging from the molding duct can be cut into individual boards without damage, and the boards must be so easy to handle that they can be transported into storage rooms for the final setting. In order to be able to produce boards economically, the setting times and stay periods respectively in the molding duct must not exceed about 5 to 6 minutes.
Under these conditions, it was heretofore possible to work only with magnesia cement, whose setting can be sufficiently accelerated by heat supply in the molding duct. Portland cement, on the other hand, sets much too slowly; and it is possible only to a limited extent to accelerate the setting by heat supply, particularly in the case of wood as a base material, where a great delay of the solidification process is caused by the active wood substances. The addition of accelerators, such as calcium chloride, does not permit either magnesia cement or portland cement to achieve even an approximately adequate setting time.